Porous carbon materials are widely used as adsorbents, catalyst supports, and filtration membranes due to their high surface area and well defined porosity. Nano-porous carbon membranes have been recently developed for gas separation through a Selective Surface Flow (SSF) mechanism. Such materials are disclosed in U.S. Pat. No. 5,104,425.
According to U.S. Pat. No. 5,104,425 the carbon membrane is made by pyrolysis of poly(vinylidene chloride-methyl methacrylate-acrylic acid-methyl acrylate) co-polymer (PVDC latex) films pre-coated on mesoporous substrates (either tubes or honeycomb matrices) under nitrogen. In the coating process, well dispersed latex particles (average diameter around 0.1 micron) in water are slip-cast on the porous substrates. In the slip-casting process, the liquid from the latex dispersion passes into the pores of the substrate while the dispersed latex is effectively filtered out by the narrowness of the pore openings and is deposited as a coating on the substrate channel surface. After drying, the well-packed latex particles on the surface are fused by heating to form a continuous polymer film and then convert to a flaw-free carbon membrane by further heating at higher temperatures. If the pores in the substrate (0.4-0.8 micron) are much larger than the latex particles, it is inevitable that latex particles go in the mesopores of the substrates during slip-casting. This may be highly undesirable since the microfiltration of polymer latex particles causes narrowing of the egress path, hence increasing the resistivity for gases passing through the matrices. As a remedy to this problem, a support layer is applied to substrate which has a pore size smaller than that of the substrate but larger than that of the membrane to be formed. Therefore, without use of the intermediate size support layer, larger pore size substrates cannot be used to hold membranes.
Another problem with the current state of the art slip cast coating process is the difficulty of controlling both coating thickness and uniformity for high channel density honeycomb matrices. As the channel density is increased, the volume of mesopores in the substrate per unit surface area of membrane decreases, and hence the volume of PVDC latex dispersion which can be slip-cast onto the channel surfaces decreases. Once the pores are entirely filled, little or no additional coating takes place and the already deposited coating may begin to slough off or re-disperse. Therefore, it is difficult to form high quality, flaw-free membranes on high channel density honeycomb matrices.
With geometrically complex honeycomb matrices having high channel density, egress path is an issue. To maximize the membrane efficiency, large mesopores are preferred in the substrate. In general, substrates with large pores have rough surfaces which are difficult to coat, affecting the slip-cast coating procedure and the quality of the carbon membrane as well.
There are other difficulties with the slip-cast coating method. The compatibility of substrate materials with the highly acidic dispersion is one problem. Basic materials may partially dissolve in the coating process. Dispersion corrosiveness and waste solution disposal are other problems.
It would be highly desirable and an advancement in the art to produce a coating on the substrate without the above problems.